This invention relates to a process for preparing olefin polymers using novel deactivated catalyst neutralization agents. Olefin polymers such as polyethylene and polypropylene are often produced by use of a transition metal containing polymerization catalyst thereby leaving small amounts of catalyst residues in the polymer. Such residues are typically deactivated to prevent continued polymerization outside the reactor. Deactivation of active transition metal catalysts results in generation of considerable amounts of acidic compounds and halide containing compounds such as hydrogen chloride. This can result in color body formation in the polymer and corrosion of processing equipment. Neutralization of such contaminants is generally achieved by incorporating a small quantity of an acid neutralizing compound, such as alkaline earth metal salts of a weak acid, for example a fatty acid calcium salt such as calcium stearate, into the polymer. This is normally accomplished by mixing the additive with the mixture exiting a polymerization reactor prior to devolatilization or polymer recovery.
A wide variety of catalyst deactivating agents have been utilized or disclosed in the prior art, including CO or CO2, and hydroxyl or carboxyl group containing compounds such as water or water releasing compounds, optionally in combination with a non-volatile component, selected from alcohols, phenols, diols, polyols, saccharides, ethers, epoxides, aldehydes, ketones, carboxylic acids, polyacids, and acid anhydrides or esters. Suitable neutralizing compounds include arnines; fatty acid salts; and salts of poly(oxyalkylene) compounds. References disclosing the foregoing compositions include EP-67,645, EP-71,252, GB-2,132,214, EP-A-140131, WO92/14766, and WO03/55920.
Disadvantageously, previous neutralizers have proven to be susceptible to decomposition or oxidation over prolonged periods. In addition, certain of the foregoing compounds, including aliphatic carboxylic acid salts or hydrolysis products thereof, are generally relatively insoluble in the polymer, resulting in decreased effectiveness. This requires use of larger quantities of additive to achieve the desired benefit, resulting in increased costs. In addition, such additives tend to separate from the polymer and migrate to the surface upon subsequent melt forming processes where they interfere with desired surface properties or esthetics of the polymer article, react with additives or other components of desired polymer blends or compositions, or plate out on processing equipment. In addition, carboxylic acid salts are relatively volatile and tend to condense within the devolatilizer and associated equipment, leading to corrosion and fouling or plugging and the need for frequent cleaning of reactor recycle and post production equipment.
Antioxidants, including phosphoric- or phosphonic acid salts, are known additives for protecting organic materials, including paints, cosmetic formulations, foodstuffs, and polymers from oxidation. Numerous such compounds and combinations of compounds are available commercially and disclosed in the art. Examples include: tris(2,4-di-tert-butylphenyl)phosphite (Irgafos™ 168), tris(nonylphenyl)phosphite, tetrakis(2,4-di-tert-butylphenyl)[1,1-biphenyl]4,4′-diylbisphosphonite (IRGANOX™ P-EPQ), and calcium bis(((3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl)methyl)ethylphosphonate) (IRGANOX™ 1425).
In general, antioxidants are a class of additives that are used to retard degradation of organic materials, formulations and natural products due to oxidation. With regard to polymers lacking the protection of antioxidants, loss of molecular weight, brittleness, discoloration, crosslinking, and deterioration of other polymer properties may occur. One mechanism of such degradation, it is believed, involves generation of free radicals, formed as a result of heat, ultraviolet radiation, mechanical shear, or impurities in the polymer. One possible pathway involves a series of reactions by means of which the free radical reacts with oxygen to form a peroxy radical, which then reacts with an available hydrogen atom of the polymer to form an unstable hydroperoxide and regeneration of a free radical. In the absence of an antioxidant, these reactions become self-propagating, and lead to polymer degradation.
Antioxidants are thought to operate according to two basic principles, referred to as primary and secondary. Primary antioxidants intercept and stabilize free radicals and peroxy radicals by donating active hydrogen atoms in preference to hydrogen atoms of the desired product. Hindered phenols and aromatic amines represent the two main types of such primary antioxidants. Secondary antioxidants prevent formation of additional free radicals by decomposing the above referenced hydroperoxides into thermally stable, non-radical, non-reactive products by means of an efficient alternative to thermolysis and generation of free radicals. Phosphites and thioesters are examples of functionality operating as a secondary antioxidant. Primary and secondary antioxidants are often used in combination, and certain compounds contain both functionalities in the same molecule, thereby combining primary and secondary antioxidizing properties into one compound.
It would be desirable if there were provided suitable additives that could be added to polymer products prior to devolatilization in order to obtain the properties of prior art acid neutralizing additives as well as anti-oxidation additives. In particular, the attainment of equivalent or improved performance in the neutralization of acidic catalyst residues with reduction or elimination of adverse consequences in their use and simultaneous incorporation of antioxidation properties in the resulting polymer is desired. Reduction of corrosion, plugging and/or fouling of devolatilization equipment and associated conveying devices due to build up of catalyst neutralization residue, while at the same time imparting antioxidant properties to the polymeric composition and elimination of some or all of the adverse consequences of conventional acid neutralizing compounds, is an achievement greatly to be desired.